One convenient method of recovering metal values from underground ore deposits incorporates the in-situ leaching of the deposit. A lixiviant or leach solution that will dissolve the metal is pumped or injected into the underground deposit. The enriched or pregnant leach solution containing the dissolved metal value is raised up above ground where the metal value is recovered. After the recovery, the now barren solution is typically reinjected into the deposit. The ability to essentially continuously recirculate or recycle the leach solution is at least eminently desirable if not necessary in such in-situ solution mining schemes. To maintain the efficacy of the spent leach solution, fortification with the solubilizing compounds will be necessary at least periodically.
Several problems attend the in-situ solution mining techniques and militate against the success of the technique. The leaching solutions often react with minerals other than the desired metal values so that the solutions become contaminated with undesired materials. The dissolved contaminants may be difficult to separate from the dissolved desirable metal values. Substantial amounts of expensive leaching compounds may be lost beyond recovery in the underground deposit. Long continuous recirculation of leach solutions may adversely affect the aquifer because ions may be captured by the mineral deposits. These ions may be retained in or near the mining zone long after the mining operation is terminated and be released to the aquifer to adversely affect water quality. Extensive restoration activities may be mandated to prohibit or minimize this adverse effect. Water available at the site is a valuable resource for purposes other than mining and should not be contaminated so as to destroy its other utilities.
Underground water saturated ore bodies containing oxide, sulfide or carbonate minerals of base metals such as uranium, copper, nickel, molybdenum, rhenium, selenium and vanadium are exemplary candidates for in situ solution mining. Uranium is an example of particularly valuable base metal. The known solution mining techniques for the recovery of uranium and the other base metals may include both acid and alkaline leach solutions. When the ore deposit contains substantial amounts of calcite, the alkaline carbonate-bicarbonate lixiviant has particular advantages. Ammonium or sodium carbonate-bicarbonate lixiviants are both known to be advantageous because they are less corrosive than acid lixiviants, are more selective in dissolving and then separating the uranium from other metals. Uranium is now believed to be present in the deposit in its reduced, e.g. insoluble tetravalent form and may have to be oxidized to its soluble hexavalent form either before or during leaching.
Deposits that are suitable for in situ solution mining typically include a permeable stratuum disposed between impermeable strata. The leaching solutions can then be restricted to the permeable base metal rich stratuum. That permeable stratuum should be afterwards restored as nearly as possible to its original state. Where the permeable stratuum is capable of yielding considerable quantities of water to wells or springs, great attention and effort are directed to disturbing the original water quality as little as possible and/or restoring it to its original quality if it is adversely affected by the injected recirculating compositions. Initial or baseline measurements on the aquifer or formation waters provide data on the original water composition, e.g. baseline TDS data (total dissolved solids). After the base metal is extracted but before the in situ mining operation is considered completed, the formation water ordinarily must be restored to some acceptable level near the baseline quality.
In the previously known processes, making the initial and/or reconstituting the recirculating lixiviant have been particularly troublesome because of their perturbative environmental effects, particularly on the formation waters. Restoration to or near the original or base line levels has, in some instances, been difficult, time consuming and, therefore, expensive.